Apparatus for producing solid hydrogen

ABSTRACT

A diamond-anvil, high-pressure cell having an apertured steel gasket interposed between the opposed diamonds is lowered into a liquified hydrogen bath at cryogenic temperatures. After the liquid hydrogen permeates the cell through the viewing apertures an initial sealing pressure is applied to the cell by a remotely operated threaded arrangement sufficient to prohibit escape of the liquified hydrogen from the aperture in the steel plate which is closed by the opposed diamonds. The cell is then typically removed from the liquified hydrogen bath and introduced into a lever type pressure applicator to further increase the pressure on the hydrogen at room temperature for the observation of its effects through suitable apertures in the cell and to convert the same to solid form.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to a method and apparatus forproducing solid hydrogen and more specifically to a method forintroducing hydrogen into a diamond-anvil, high pressure cell forsubsequent high-pressure processing and the construction of the cell forcarrying out the method.

2. Prior Art

Diamond-anvil, high-pressure cells and apparatus for applying pressureto the cells are old and well known as evidenced by the U.S. Pat. Nos.to Weir et al. 3,079,505 and Kirk 3,509,597. However, the cellsdisclosed in these patents are not adapted to containing a compressedgas at high pressure between the opposed diamonds and accordingly, thepatents are silent as to any methods for introducing such a compressedgas into the cells.

The use of an apertured steel gasket between opposed diamond anvils todefine a sample cell was disclosed in the "Annual Report of theDirector" of the Carnegie Institution Geophysical Laboratory for theyear July 1, 1977--June 30, 1978 which was issued in December 1978.While the gasket disclosed in this publication can be used to confineliquid or solid samples such samples were placed in the cell by methodswhich were totally unsuitable for introducing a gas such as hydrogen.

U.S. Pat. No. 3,521,457 to Hemstreet relates to apparatus for makinghydrogen "slush." The product formed differs substantially from thatherein formed and the process steps bear little similarity to those ofthe present invention, the "slush" in fact being formed at lowtemperature using a "scraping" technique.

U.S. Pat. No. 3,521,458 to Huibers et al. is, in overall substance,cumulative to the Hemstreet patent.

SUMMARY OF THE INVENTION

The present invention provides a unique method and apparatus for theintroduction of hydrogen into a diamond-anvil, high-pressure cell.

The method according to the present invention involves the introductionof a new and improved megabar cell having remotely operated pressuremeans into a bath of liquified hydrogen prior to moving the diamondanvils into pressure engagement with a stainless steel gasket. Theliquified hydrogen will permeate the entire megabar cell constructionthrough the usual viewing passages and upon operation of the remotelyoperated pressure applying means, the diamond anvils will be broughtinto engagement with the stainless steel gasket with sufficient pressureto sealingly trap a quantity of liquified hydrogen in an aperture orsample chamber located in the gasket between the two diamond anvils. Themegabar cell with the pressure applying means still attached thereto isthen generally removed from the liquified hydrogen bath and placed in alever press for the application of further pressure to the liquifiedhydrogen at room temperatures for study of the liquified hydrogenthrough suitable viewing ports in the cell.

The modified megabar cell according to the present invention iscomprised of a piston and cylinder arrangement having adjustable meansfor supporting two diamond anvils in opposed relation to each other formovement toward and away from each other. A stainless steel gasket issuitably mounted within the cylinder and is provided with an aperture inalignment with the diamond anvils. A thrust block is secured to the openend of the cylinder by screw means and bears against the end of thepiston which protrudes from the cylinder so that upon rotation of thescrew means into the cylinder the piston will be forced into thecylinder thereby forcing the diamond anvils into engagement with opposedsurfaces of the gasket surrounding the aperture therein.

The apparatus for filling the aperture in the gasket between the diamondanvils with a liquid hydrogen is comprised of a vertically disposed opentopped double walled elongated evacuated cylinder. The space between thedouble walls is completely filled with liquid nitrogen and a smallamount of liquid helium is located in the bottom of the cylinder and avessel containing liquid hydrogen is suspended within the cylinderimmediately above the liquid helium. The megabar cell is suspendedwithin the vessel containing the liquid hydrogen to a depth sufficientto submerge the viewing apertures thereby allowing liquid hydrogen toflow into the interior of the megabar cell. Remotely operated rod meansare provided for rotating the screw means extending through the thrustblock to force the thrust block toward the cylinder thereby forcing thepiston into the cylinder to trap a quantity of liquid hydrogen withinthe aperture in the gasket between the diamond anvils.

The present invention also relates to a process for producing solidhydrogen which comprises: introducing liquid hydrogen intopressure-application means at cryogenic temperatures; sealing saidliquid hydrogen into said pressure application means at cryogenictemperatures by the application of pressure thereto; maintaining saidapplied pressure; and increasing the pressure on said liquid hydrogenuntil said liquid hydrogen converts to solid form.

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of a preferred embodiment of the invention as illustrated inthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded view partly in section showing the megabar cell,thrust block and screw means according to the present invention.

FIG. 2 is a spatial view of the half cylinders for supporting thediamond anvils and their relative degrees of freedom.

FIG. 3 is a sectional view of the megabar cell according to the presentinvention.

FIG. 4 is an exploded detail view of the apertured gasket and itsrelationship to one of the diamond anvils.

FIG. 5 is a schematic view of the apparatus according to the presentinvention for filling the megabar cell with liquid hydrogen.

FIG. 6 is a side elevation view of the megabar cell with the thrustblock according to the present invention located within a lever typepressure applying means.

FIG. 7 is an exploded view of the lever press showing the relation ofthe parts thereof to the megabar cell.

DETAILED DESCRIPTION OF THE INVENTION

The megabar cell 10 according to the present invention is comprised of acylinder 12 having a piston 14 slidably mounted therein. The piston 14is of sufficient length so that its lower end protrudes beyond the openend of the cylinder in the operative position which will be describedhereinafter. Semicylindrical support rockers 16 and 18 are located incomplimentary recesses in the bottom of the cylinder 12 and the opposedsurface of the piston 14. The support rockers are each adjustable aboutaxes of rotation which are disposed orthogonal to each other. Therockers are constructed of tungsten carbide and are each provided with ahole or slot 20, 22 for experimental purposes involving X-raydiffraction techniques. A thin foil of zirconium metal (not shown) maybe located between the rockers and the piston and cylinder to cushionthe contact of the tungsten carbide rockers with the hardened tool steelof the piston and cylinder. Set screws 23 and 25 are located in twopairs of apertures 24, 26, only one of each pair being shown in FIG. 1,for bearing against the opposite ends of each rocker to secure therockers in their adjusted positions. Two diamond anvils 28 and 30 aresecured to the upper and lower rockers 16 and 18, respectively, with anepoxy resin. The axis of the diamonds are aligned with the apertures 20and 22 which in turn are aligned with apertures 32 and 34 in thecylinder and piston, respectively. Additional viewing apertures 36 and38 are provided through the side walls of the cylinder 12 in alignmentwith the diamond anvils in order to carry out the precise adjustment ofthe rockers about their axes to achieve parallelism and to match theanvils. The detailed alignment procedure will be described in detaillater in the present application.

The cylinder 12 is provided with an enlarged flange 40 extending aboutthe end thereof through which the piston protrudes. Four equidistantthreaded apertures (not shown) are located in the end of the flange 40extending parallel to the axis of the piston and cylinder arrangement. Athrust block 42 is provided with a central aperture 44 which is disposedin alignment with the aperture 34 in the piston 14 and the annular endsurface of the piston 14 is disposed in abutting relation with thesurface of the thrust block 42 surrounding the aperture 44. Fourequidistant smooth bores 46 are located about the central aperture 44and are disposed in alignment with the threaded apertures in the flange40 of the cylinder 12. Four screws 48 are each provided with a threadedportion 50 adapted to be threaded into the threaded apertures in thecylinder, a smooth shank portion 52 adapted to be located within thesmooth bores 46 in the thrust block 42 and a knurled enlarged headportion 54. One or more Belleville springs 56 are located on the smoothshank of each screw 48 so that as the screws are threaded into the endof the cylinder 12 an axially directed force is applied by the heads 54to the piston 14 through the Belleville springs 56 and the thrust block42. Diametrically opposed screws are threaded in opposition directionsso that as the screws are rotated there will not be any rotationaltorque applied to the piston and cylinder assembly.

As best seen in FIGS. 2 and 3 a gasket 60 of T301 full-hardenedstainless steel is loosely supported between the diamond anvils 28 and30 by means of blocks 62 and screws 64 threaded into the blocks and theend of the piston through apertures in the ends of the gasket 60. Thegasket is provided with an aperture 66 which is in alignment with thepressure face 68 of each anvil 28 and 30. The aperture in the gasket issurrounded by an annular extruded ridge of metal 70 having inner slopingsurfaces 72 and 74 on opposite surfaces thereof corresponding to thesurfaces of the diamonds 28 and 30. The annular ridge 70 is formed as aresult of the pressure application of the diamonds to the gasket as iswell known in the art. Thus, upon pressure engagement of the diamondswith the gasket, a sample cell will be defined by the wall of theaperture 66 and the pressure surfaces 68 of the diamond anvils 28 and30.

The apparatus for filling the sample cell with hydrogen is comprised ofan elongated evacuated double cylindrical vessel 78 having an outer wall80 and an inner wall 82 spaced therefrom to define a chamber for thereception of liquid nitrogen. The inner chamber defined by the wall 82is filled with a small amount of liquid helium. The upper end of theinner chamber is sealingly connected to and closed by a plate 84 whichis suspended from any suitable support by means of rods 86 and 88. Asecond plate 90 is secured to and supported by the rods 86 and 88 inspaced parallel relation above the plate 84 and supports the knurledheads 92 of control rods 94 which rotatably extend through the plates 90and 84. A hollow central supporting rod 96 is secured to the plates 84and 90 and extends through these plates into the chamber defined by theinner wall 82 of the vessel 78. The lower end of the hollow support rod96 is threadedly secured into the central aperture 44 of the thrustblock to support the megabar cell 10, the construction of which is shownin detail in FIG. 1. The upper ends of the knurled heads of the screws48 are each provided with a hexagonal recess 49 suitable for thereception of an Allen head wrench. The lower ends of the control rods 94are provided with a hexagonal cross-sectional configuration and areadapted to be received within the recesses in the heads 54 of the screws48. The upper ends of the knurled heads 92 of the control rods 94 arealso provided with hexagonal recesses so that the control rods mayeither be rotated manually or by means of an Allen head wrench.

Hydrogen gas is supplied from a pressurized tank 98 and is pumpedthrough a liquid nitrogen trap 100 into a cup shaped vessel 102 throughthe flexible conduit 104 which extends downwardly through the hollowsupport rod 96 to the interior of the vessel 102. Suitable thermocouplewires 106 also extend down through the hollow support rod 96 and sensethe interior and exterior temperatures of the vessel 102. The vessel 102may be detachably secured to the megabar cell 10 by any suitable means(not shown) or may be supported from the plate 84 independently of themegabar cell 10.

After the liquid hydrogen in the vessel 102 permeates the entire cell 10the screws 48 are rotated to bring the diamond anvils 28 and 30 intoengagement with surfaces 72 and 74 surrounding the aperture 66 in thegasket 60 to trap a quantity of liquid hydrogen within the aperture 66.The cell 10 is then removed and placed in a lever-type press 110 asshown in FIGS. 5-7.

The press 110 is comprised of a main block 112 having a frontrectilinear notch 114 adjacent to but spaced inwardly from one end ofthe block and a second rectilinear notch 116 at the opposite end of theblock. A cylindrical aperture 118 is located in the bottom of the notch114 and extends entirely through the block 112. The cylinder 12 of thecell 10 is located in the aperture 118 with the thrust block 42 restingin the bottom of the notch 42.

A pair of identical levers 120 are each provided with notches 122, 124at opposite ends thereof and a hole 126. The levers 120 are disposed onopposite ends of the block 112 and are supported on trunnions 128 whichprotrude from opposite ends of the thrust block 42. The fulcrum pins 130located adjacent the notch 112 engage the notches 122 of the levers. Apressure block 132 have a pair of projections 134 is located in thenotch 116 with the projections 134 engaging the notches 124 of levers120. A bolt 136 extends through a smooth base 138 in the pressure block132 and is threaded into a threaded aperture 140 in the bottom of notch116 in block 112. A plurality of Belleville spring washers 142 surroundthe bolt 136. Upon tightening of the bolt the pressure block 132 willpivot the ends of the lever 120 in contact therewith counter-clockwiseabout pins 130 as shown in FIG. 5 to force the piston 14 into thecylinder 12 to increase the pressure on the hydrogen in the gasket.

Where processing is carried out entirely at cryogenic temperature, ofcourse conventional remote control means are used for all manipulations;in distinction, where final pressure elevation is at room temperature orabove, such are not required.

Turning to the process of the present invention in more detail, asearlier set forth it broadly comprises four steps:

1. Introducing liquid hydrogen into pressure-application means asearlier described at cryogenic temperature, i.e., at about 17° K.;

2. Sealing the liquid hydrogen into the pressure-application means atcryogenic temperature by applying pressure thereto;

(As will be appreciated by one skilled in the art, a sample chambercontaining the liquid hydrogen must be maintained at a pressure slightlygreater than that applied to the liquid hydrogen to prevent leakagethereof.)

3. While substantially maintaining the pressure above mentioned, theliquid hydrogen is then generally brought to approximately roomtemperature; (As will be appreciated by one skilled in the art,temperatures other than room temperature can be used, but consideringthe following processing step it is most convenient to operate at roomtemperature.) and

4. Finally, once the liquid is at approximately room temperature,increasing the pressure until the liquid hydrogen converts to solidform.

In the context of the process of the present invention, the"maintaining" of the pressure in step 3. above is to insure there is nohydrogen leakage. Obviously, one could merely seal and continue directlywith presure elevation without a separate, distinct "maintaining" step.

While the process of the present invention involves several relativelystraight forward process steps, there are several criteria which must beobserved in our currently preferred modes of practicing the invention,and these are set forth below.

First, the extremely high pressures necessary to practice the presentinvention are most conveniently obtained by using a diamond anvil pressas earlier described, i.e., involving the use of two diamond anvils (0.3Caret brilliant cut diamonds; octagonal anvil faces ground flat orbeveled to 1˜2° with low strain birefringance: 2 to 10×10⁻⁵) spaced by astainless steel gasket (thickness: 250 μm) which contains the liquidhydrogen sample of the present invention. The stainless steel gasket iscommercially obtained from Ulbrich Stainless Steels & Special Metals,Inc. under the name T301 Full Hardened stainless steel, the gasketbeing, of course, work hardened.

In actuality, we initially begin with two gaskets which are essentiallyidentical except one gasket has a sample chamber (which will be used inthe actual process run) and the second gasket does not have a samplechamber; this second gasket is used for initial calibration of thediamond anvils to ensure precise alignment to avoid diamond fracture.

After the above procedure, the following optional alignment may beperformed. The gasket is removed and solid, powdered AgI is coated on tothe faces of the diamond anvils in thin layer form, i.e., about 10microns thick. Pressure is then applied to the diamond anvils to bringthe faces thereof into contact and the phase transition of AgI observed.AgI I converts to AgI II at 3 Kbars and AgI II converts to AgI III at100 Kbars, and thus we typically apply pressure in excess of 100 Kbarsduring this step of the alignment procedure.

The Becke line, which reflects a difference in phase density, isvisually observed, and by observing the Becke line the diamond anvilfaces can be brought into precise alignment. If necessary, during thisalignment step the carbide rockers can be used to gain a more precisealignment of the diamond anvil faces, with our goal being substantiallyzero torque on the diamond anvil faces during active processing.

We first align the diamond anvils as closely as possible visually, i.e.,until the last light interference fringe disappears, the gasket withoutthe sample chamber being inserted between the diamond anvils, the upperface of the gasket containing a thin layer of ground ruby powderslightly less than 10 microns in thickness, the powder being ground to10 microns or less in size. The ground ruby powder is Al₂ O₃ doped with0.55% Cr.

It is to be noted that while we use single crystal diamonds, we believefor commercial processing other pressure application means will probablybe used due to cost considerations, e.g., polycrystalline diamonds andthe like.

Next, pressure (e.g., up to 700 Kbars) is applied to the gasket by thediamond anvils and the ground ruby powder is excited with a He-Cd laser(542 nm) with increasing pressure.

The frequency of the fluorescent emission of the rubies changes withpressure (the ruby R₁ fluorescent line), and, accordingly, from apre-prepared calibration curve, any pressure gradient across the gasketcan be measured and, if the pressure gradient is not symmetrical to theaxis of the anvils the diamond anvils can be realigned by realigning thecarbide rockers.

Following the above procedure, the gasket with the sample chamber isinserted between the diamond anvil faces and processing is begun.Typically, we indent the gasket and then drill through the gasket toform the sample chamber which will be sealed by the opposing diamondanvils upon pressure application The gasket, though work hardened asobtained, is, we believe, further work hardened during the pressureapplication according to the present invention and we believe thisassists in achieving the desired sealing effect. Specifically, in thepressure range 50 to 200 Kbars the gasket thins to 10-20 μm between theanvil faces.

The first step in the processing is to insert the pressure applicationmeans into cryogenic apparatus as earlier described, i.e., liquid N₂bath surrounding a liquid He bath above which is a container of liquidH₂. Thermocouples are utilized to ensure that the pressure applicationmeans is disposed at a proper location in the bath so that liquidhydrogen is introduced into the sample chamber, actually the samplechamber being filled to overflowing with liquid hydrogen. A fewmicrograms of small ruby crystals (10-20 μm in diameter) are alsopresent in the sample chamber to permit pressure measurement using theNational Bureau of Standards technique with reference to theprecalibrated pressure shift of the ruby fluorescent R₁ line.

It is to be noted that we use rubies for pressure measurements becauseof the availability of the National Bureau of Standards calibrationcurves. Other materials could, of course, be used but it would benecessary to go to the expense of generating calibration curvestherefore.

In a similar fashion, gaskets other than T301 stainless steel can beused, so lone as the desired sealing effects at the high pressure ofoperation are obtained.

Once the liquid hydrogen fills the sample chamber (250 μm in diameter),pressure is applied to the sample chamber at cryogenic temperatures,i.e., at around 17° K.

The metal of the gasket, with the advancing pressure, will undergoplastic flow with the result, if diamond alignment is precise as abovedescribed, that a sealing area of the metal gasket surrounding thesample chamber will have a pressure of at least 10 to 20% higher thanthat on the liquid hydrogen in the sample chamber. While this pressuredifferential is not critical, we have found the above pressuredifferential to be adequate for our purposes in ensuring no liquid H₂leakage.

Typically, the pressure applied to the liquid hydrogen sample at thisstage is on the order of 20 Kbars.

While the exact pressure applied during this stage of the process is notoverly critical except to ensure that the liquid hydrogen is sealed inthe sample chamber without possibility of substantial escape, based onour current work we believe that the pressure applied in the cryostatshould be on the order of about 40 to about 60 Kbars.

The pressure application means is then typically removed from thecryostat and is generally permitted to rise in temperature; while thetemperature need not be room temperature, it is much easier to work withthe processing apparatus at room temperature since remote control meansas are required for processing at cryogenic temperature are no longernecessary.

Our current results indicate that temperatures of greater than 17° K. toabout 4,000° K. can be used during final pressure elevation, but asindicated processing is most generally at about 298° K. If finalpressure elevation is in the cryostat, the temperature is above 17° K.For high temperature pressure elevation, a laser can be used to heat thesample; in fact, operation at high temperature (with laser heating) mayprove to be the most effective way to obtain metallic hydrogen.

In this regard, while 57 Kbars at 298° K. provides solid hydrogen,lesser final pressures are sufficient to obtain solid hydrogen attemperatures below 298° K. but above 17° K. while higher final pressuresare needed at above 298° K. At present, we do not raise to finalpressure at 17° K., rather, the temperature must be greater than 17° K.at the time of final pressure elevation.

Hereafter, we merely discuss the invention in terms of such "roomtemperature" processing.

After the liquid hydrogen is at room temperature, the pressure obtainedin the cryostat still being substantially maintained, the pressure onthe liquid hydrogen is then increased at room temperature, with theRaman spectra of the liquid hydrogen sample being optically observed aspressure is increased.

Once the pressure reaches 57 Kbars, the hydrogen converts to clearcrystals (solid), and will maintain this form until the release ofpressure, whereafter the hydrogen will revert from the solid to thefluid to the gaseous form.

At this stage, the product of the present invention can be used in anyapplication where a dense, compact form of hydrogen is needed andelevated pressure environments are available, for example, as a rocketfuel, as an explosive, as a source of hydrogen for fusion and the like.

As the pressure is increased over 57 Kbars, the clear hydrogen crystalsmerge into solid form without grain boundaries and the refractive indexincreases. At 360 Kbars the visible boundary line between the rubycrystals and the solid hydrogen disappears; the specific gravity of thesolid hydrogen is calculated to be about 0.6˜0.7. From 360˜650 Kbars therefractive index of the solid hydrogen continues to increase.

If the pressure is further increased, for example, to pressures on theorder of 1 Megabar or higher, it is our belief that the resulting solidhydrogen will invert to the metallic state and will remain in that stateeven at reduced pressures.

In this last condition, the density of the solid hydrogen is very highand, in addition to the uses above described, it is our belief that thesolid hydrogen will be a highly efficient super conductor. In thislatter state, we believe the metallic hydrogen will have a specificgravity of about 1.0.

Based on calculations, solid hydrogen of a density of about 1 g/cc willhave an extremely high specific impulse on the order of 1,400 seconds,far superior to materials such as conventionally used JP-4 and LOX,which have specific impulses of about 400 seconds.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose in the art that various changes in form and details may be madetherein without departing from the spirit and scope of the invention.

We claim:
 1. An apparatus for filling a cell means defined by anapertured gasket and two opposed diamond anvils with hydrogen comprisingcold chamber means, vessel means, means for supporting said vessel insaid chamber means, means for supporting said cell means in said vesselwith said diamond anvils spared from said gasket, means for supplyingliquid hydrogen to said vessel whereby said cell would be filled withliquid hydrogen and remotely controlled means for moving said diamondanvils into engagement with said gasket about the aperture therein totrap an amount of liquid hydrogen in said cell means.